INDIANAPOLIS — A federal judge struck down Indiana's ban on same-sex marriage Wednesday in a ruling that immediately allows gay couples to wed.

U.S. District Judge Richard Young ruled that the state's ban violates the U.S. Constitution's equal-protection clause because it treats couples differently based no their sexual orientation.

"Same-sex couples, who would otherwise qualify to marry in Indiana, have the right to marry in Indiana," he wrote. "These couples, when gender and sexual orientation are taken away, are in all respects like the family down the street. The Constitution demands that we treat them as such."

The clerk in Marion County, home to Indianapolis, says the office will start issuing marriage licenses immediately.

The Indiana attorney general's office said it would appeal the ruling but declined further comment.

The ruling involves lawsuits filed by several gay couples, who along with the state had asked for a summary judgment in the case. Young's ruling was mixed but was generally in favor of the gay couples and prevents the state from enforcing the ban.

Federal courts across the country have recently struck down gay marriage bans, but many of those rulings are on hold pending appeal. Attorneys on both sides of the issue expect the matter to eventually land before the U.S. Supreme Court.

"Indiana now joins the momentum for nationwide marriage equality and Hoosiers can now proclaim they are on the right side of history," Lambda Legal, the national gay rights group that represented five of the couples, said in a statement Wednesday.

A movement to add a gay marriage ban to the Indiana constitution faltered during this legislative session when lawmakers removed language about civil unions from the amendment. That means the soonest the issue could appear on a ballot is 2016, unless federal court rulings scuttle the proposed amendment.